It is important that any refrigeration system have a sufficient or adequate charge of refrigerant to accomplish the function for which the refrigeration system was designed. A typical refrigeration system is required to cool a conditioned space to a predetermined temperature selected from a relatively narrow temperature range, and the typical refrigeration system is operated within an ambient temperature which also varies within a predetermined relatively narrow range. Thus, regardless of the method used to test adequacy of refrigerant charge, it is relatively easy to duplicate the operating ambient and to set the objectives to be accomplished in the space to be conditioned.
A transport refrigeration system, such as the refrigeration systems used on trucks, trailers, and reefers, is not a typical refrigeration system. It is not a typical refrigeration system because it is required to hold the temperature of a served space to any selected set point temperature in a wide range, such as to a selected set point temperature within a range of -20.degree. F. to +80.degree. F. (-29.degree. C. to +26.7.degree. C.), with ambient temperatures which range from winter to summer in different parts of the world. Determining sufficiency of refrigerant charge in a transport refrigeration system is thus difficult because it is not easy to reproduce the most severe operating and ambient temperature conditions which a transport refrigeration system must be able to accommodate.
The traditional method of verifying adequate refrigerant charge in a transport refrigeration system, such as a refrigerated truck, trailer, or container (reefer) involves the steps of cooling the associated cargo and cargo space to a low set point temperature, such as 0 degrees F. (-17.8.degree. C.), while under actual or simulated high ambient temperature conditions, eg., 150 pounds compressor head pressure. This method confirms that the system has sufficient refrigerant charge to operate under this extreme condition.
The traditional method, however, is very time consuming, due to the broad range of possible set point temperatures and the large thermal mass of a refrigerated transport refrigeration system, and the traditional method is only practical when the load in the associated conditioned space will not be damaged by the deep cooling of the conditioned cargo space. Further, the traditional method will not detect an over-charge condition, which may damage the refrigerant compressor due to excessive head pressures.
It would be desirable, and it is an object of the present invention, to provide new and improved methods for more rapidly determining whether the refrigerant charge in a transport refrigeration system is within an acceptable range, before removal of the transport refrigeration system from a terminal where inadequate charge, or an over-charge condition, may be remedied.
It is also an object of the invention to provide such more rapid refrigerant charge determining methods which are applicable to transport refrigeration systems which are already loaded with cargo to be conditioned, without danger of damaging the cargo, regardless of the required set point temperature.